1. Field of the Invention
The present invention relates generally to the generation of steam/electricity from a carbonaceous material and the production of activated carbon. Appropriate combination of these two processes provides significant cost savings through requiring fewer pieces of equipment, reducing operating costs, and increasing efficiency between the two processes. A portion of coal processed for a steam/electricity generation plant is diverted to a carbon activation plant thereby reducing equipment costs. A portion of steam produced in the steam/electricity generation plant is directed to the activated carbon plant for steam activation of a carbonized product. A portion of the combustible gases resulting from the carbonization and activation steps of the activated carbon plant is directed back to the steam/electricity plant, e.g. for reburn, and a portion may be recycled for use in the carbonization and/or activation steps. Activated carbon product resulting from the activated carbon production plant may be used, for example, to reduce heavy metal (e.g. mercury) emissions and/or to control NOx emissions in power plant flue gas, for example, coal-fired power plant flue gas, by contacting the NOx-containing flue gas with activated carbon thereby converting NO to N2.
2. Background of the Invention
Carbon-based sorbents, including activated carbon, are currently used for controlling vapor-phase mercury emissions in coal-fired power plant flue gases. In a typical application, carbon sorbents are injected into the flue gas duct upstream of a particulate removal device such as a fabric filter or an electrostatic precipitator. The activated carbon used for such injection is typically manufactured off-site from carbonaceous materials like coal or coconut shells.
In existing stand-alone activated carbon production plants, an associated boiler generates steam for activating a carbonized material. The typical activated carbon plant generates, from both carbonization and activation steps, product gases which may comprise, for example, hydrocarbons, carbon monoxide, hydrogen, ammonia, hydrogen cyanide, hydrogen sulfide, and combinations thereof. Heat generated from combustion of these gases may be used to make steam in the on-site boiler. The steam may then be directed back to the carbonization and/or activation steps. Depending on regulatory requirements, the resultant flue gas from the boiler must be cleaned to varying degrees before discharge into the atmosphere via a stack.
The current use of separate systems for activated carbon production and energy production is not optimal, requires separate energy production for the operation of duplicate processes, and produces significant pollution as a result of the energy production.
Accordingly, there is an ongoing need for a system and method for the coproduction of activated carbon and steam/electricity.